This invention relates to an image forming apparatus such as a digital copying machine or laser printer for scanning and exposing a single light beam or a plurality of light beams emitted from a semiconductor laser on a photosensitive drum surface to form a single electrostatic latent image on the photosensitive drum and more particularly to a light beam scanning apparatus provided on the image forming apparatus for scanning the single light beam or the plurality of light beams.
In recent years, various types of digital copying machines for forming an image by scanning and exposing the light beam and using the electrophotographic process are developed. Recently, in order to further enhance the image forming speed, a multi-beam type digital copying machine in which a plurality of light beams are emitted to simultaneously scan a plurality of lines by use of the plurality of light beams has been developed.
The multi-beam type digital copying machine includes an optical system unit as a light beam scanning apparatus having a plurality of semiconductor laser oscillators for emitting light beams, a polygonal rotating mirror such as a polygon mirror for reflecting the light beams output from the plurality of semiconductor laser oscillators towards the photosensitive drum and scanning the photosensitive drum by the light beams and a collimator lens and f-θ lens as main components.
Conventionally, in the above multi-beam type digital copying machine, control of the exposure position of the light beam in the main scanning direction and control of the exposure position in the sub—the main scanning direction (that is, control of the passage position of the light beam) are effected in the optical system unit in order to form an image with high image quality. A concrete example of the above technique is disclosed in, for example, Japanese Pat. Appln. KOKOKU Publication No. 1-43294, Japanese Pat. Appln. KOKOKU Publication No. 3-57452, Japanese Pat. Appln. KOKOKU Publication No. 3-57453, Japanese UM. Appln. KOKOKU Publication No. 5-32824, Japanese Pat. Appln. KOKAI Publication No. 7-72399, Japanese Pat. Appln. KOKAI Publication No. 7-228000, Japanese Pat. Appln. KOKAI Publication No. 9-210849, Japanese Pat. Appln. KOKAI Publication No. 9-258125, Japanese Pat. Appln. KOKAI Publication No. 9-314901 and Japanese Pat. Appln. KOKAI Publication No. 10-76704. However, the techniques disclosed in the above publications have the following problems.
That is, for control of the light beam exposure position in the main scanning direction, it is important to mount a light beam detecting device constructed by a plurality of optical sensors in a preset direction with respect to the main scanning direction of the light beam. That is, if the light beam detecting device is mounted in an inclined state, it becomes impossible to correctly detect the light beam position in the main scanning direction and, for example, there occurs a problem that a vertical line cannot be drawn straight.
However, an example indicating that the sensor itself has a function of detecting the relation between the mounting direction of the light beam detecting device and the main scanning direction of the light beam is disclosed only in Japanese Pat. Appln. KOKAI Publication No. 9-314901. Even in this example, the inclination detecting range is extremely narrow and there occurs a problem that detection and adjustment of the light beam position are difficult.
For control of the light beam position in the sub-scanning direction, examples in which the passage position of the light beam in the sub-scanning direction is replaced by time at which the light beam passes the sensor and detected are disclosed in Japanese Pat. Appln. KOKAI Publication No. 7-72399, Japanese Pat. Appln. KOKAI Publication No. 7-228000 and Japanese Pat. Appln. KOKAI Publication No. 9-210849.
However, if a variation occurs in the f-θ. characteristic of the f-θ lens mounted on the optical system unit or a variation occurs in the rotation speed of the polygon mirror, then a variation will occur in the scanning speed of the light beam on the sensor and a detection error may occur when the detection method based on the passage time of the light beam is used.
Further, in Japanese Pat. Appln. KOKAI Publication No. 9-258125, Japanese Pat. Appln. KOKAI Publication No. 9-314901 and Japanese Pat. Appln. KOKAI Publication No. 10-76704, examples in which the passage position of the light beam is driven into a portion between specified sensor patterns formed on the light beam detecting device to set the passage position of the light beam in a preset position are shown. However, with this construction, it is necessary to independently drive the light beams to the preset passage position and actuators for controlling the passage positions of the light beams are required by a number corresponding to the number of light beams. That is, in comparison with a case wherein one light beam is used as a reference and the passage positions of the remaining light beams are controlled, the number of actuators is larger by one and the cost becomes higher.
Further, if the detecting pattern for driving the light beam to the preset position is used, the precision of detection is high, but a range (detection range) in which each sensor output of the detecting pattern varies with a variation in the passage position of the light beam is narrow. Therefore, the control process becomes complicated and time for the control process becomes long.
If it is possible to control the passage position of each light beam for a plurality of resolutions, the number of sensor patterns for driving each light beam is increased and the structure of the sensor becomes complicated.